1. Field of the Invention
The present invention relates to an extruder and a method of controlling the extruder for extruding a high polymer material such as rubber to form a sheet.
2. Description of the Prior Art
This kind of extruder is provided with a roller die having a pair of upper and lower rollers on the outlet side of an extruder body. A material supplied from a batch type mixer into the extruder body is extruded by a screw provided in the extruder body and is allowed to pass between the pair of upper and lower rollers of the roller die, thus forming a sheet from the extruded material.
In such an extruder, a bank quantity of the material accumulated in a bank portion defined between the outlet of the extruder body and the roller die must be properly controlled to uniform the shape (width, thickness, etc.) of the sheet to be discharged.
A bank control system (for example, Japanese Patent Publication No. 55-4575) for controlling the bank quantity is known. According to the system, the bank quantity in the bank portion, a load applied to rollers, a bank pressure are detected by sensor equipment such as cylinder, load cell, pressure sensor, and the like when the two rollers rotate at a constant speed. A sheet is formed from the extruded material while increasing/decreasing the screw speed so that detected values are equal to set values, respectively.
However, the conventional bank control system has only a feedback control function of setting volume and pressure of the material in the bank portion to preset bank quantity target values in order to uniform the shape of the sheet. In order to continuously extrude the materials of different kinds, when a preceding material to be extruded is succeeded by a following different material to be extruded, the bank quantity in the bank portion can be set to only the target value to uniform the sheet shape.
When the materials of different kinds are continuously extruded, there is a case that the preceding material continues to remain in the bank portion and is mixed with the following material, and as a result, the following material is burnt or properties of the material vary.
In order to solve the problems, it can be considered that the smallest possible bank quantity in the bank portion is designed to be detected and the preceding material is reduced to the smallest possible bank quantity the preceding material can be succeeded by the following material, and then both of the materials are mixed.
However, since means (refer to FIG. 4 for example) for directly measuring the bank quantity in the bank portion by a cylinder, a phototube, or the like measures the level of the material in the bank portion only from the upper portion of the outlet, the small bank quantity cannot be measured.
On the other hand, means (refer to FIG. 12 for example) for detecting a load applied to rollers by a load cell and estimating the bank quantity from the detected load has the difficulty of correctly detecting the volume from the load since kinds and hardness of materials are different. Therefore, when an attempt is made to reduce the bank quantity in the bank portion as much as possible on the basis of the detection value by the load cell, it happens that no preceding material to be extruded remains in the bank portion due to a measurement error, and it causes running out of sheet.